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T-cells help the human body repair muscle damage after exercise, according to a study published in Frontiers in Physiology.

The research helps explain the “repeated bout effect” — the phenomenon in which the muscles feel less sore after repeated bouts of exercise.

Scientists already knew that the immune system plays some role in how muscle repairs itself and protects against additional damage. But the new study shows for the first time the presence of specific immune workers: T-cells.

“You think of T-cells as responding to infections, not repairing muscles — but we found a significant accumulation of T-cells infiltrating damaged muscle fibres,” said Robert Hyldahl, assistant professor of exercise science at Brigham Young University. “Our study is the first to show T-cells present in human muscle in response to exercise-induced damage.”

As part of the study, 14 men and women did two vigorous rounds of exercise on an isokinetic dynamometer machine, 28 days apart. Before and after each exercise session, the research team took muscle biopsies from the subjects and then used immunohistochemistry and microscopy to analyse the muscle tissue.

Results showed an expected increase in certain white blood cells after the second bout of exercise, but only identified the T-cells after it was suggested by Amanda Gier, one of two undergraduate coauthors on the paper, who was enrolled in an immunology course at the time, Brigham Young University said.

“T-cells, up until recently, were not thought to enter healthy skeletal muscle,” explained lead author and graduate student Michael Deyhle. “We hadn´t planned on measuring them because there´s no evidence that T-cells play a role in infiltrating damaged muscle tissue. It´s very exciting.”

According to the university, the presence of T-cells suggests that muscles become more effective at recruiting immune cells following a second bout of exercise and that these cells may facilitate accelerated repair. In other words, the muscle seems to remember the damage and reacts similarly to when the immune system responds to toxins, bacteria or viruses.

The researchers also found that inflammation actually increased after the second round of exercise, suggesting that inflammation itself probably does not worsen exercise-induced muscle damage.

“Many people think inflammation is a bad thing,” Deyhle said. “But our data suggest when inflammation is properly regulated it is a normal and healthy process the body uses to heal itself.”